This invention relates generally to the field of drainage structures, such as manholes for underground sewer pipes.
As far as known, the usual prior method of forming drainage structures, such as manholes for subterranean sewer pipes, has consisted of making an excavation in the earth extending downwardly to the sewer pipe and building or setting up forms which may be prefabricated in sections and which are positioned in the excavation, and thereafter pouring concrete in the forms to form the manhole. The excavation is generally made sufficiently large to allow a workman to enter a work space between the excavation walls and the forms for setting up the forms. Many satisfactory manholes have been foamed with this prior art method though the method involves a tedious and time-consuming job to set up the forms in the excavation. As with most excavations several feet below the ground surface, if heavy or extensive rains occur after the excavation and before or after building the forms, caving in of the dirt forming the walls of the excavation may occur and require extensive excavating and/or repairs which are very costly and wasteful. The time heretofore required for excavating and setting up stationary forms makes it difficult to complete the excavation and manhole in a single day or in a short enough period to avoid such problems caused by rainfall.